1. Field of the Invention
The present invention relates to a surface-treated aluminum material employed in various containers like beverage cans, caps and food containers, as well as deep-drawing containers and decorative boards. More specifically, the present invention improves the adhesive properties between an organic resin film and an aluminum material in the case where the organic resin film is laminated onto the surface of the aluminum material.
This application is based on a patent application filed in Japan (Japanese Patent Application No. 2000-163621), the contents of which are incorporated herein by reference.
2. Background Art
It has been the conventional practice to employ a surface-treated aluminum material in various types of containers, such as beverage cans, as well as in decorative boards used in furniture or interior building materials in which a chromate-phosphate or chromate conversion coating is formed to the surface of an aluminum material, and an organic resin film is laminated to the surface of this chromate-phosphate or chromate conversion coating.
A surface-treated aluminum material employed in a beverage can must have strong adhesive properties so that the laminated organic resin film does not peel away from the aluminum surface when the material is worked into a can. Moreover, the surface-treated aluminum material must be resistant to the corrosive effects of the material within the can. Furthermore, a surface-treated aluminum material employed in a decorative boards or the like must also have excellent resistance to corrosion over long periods of time, and strong adhesive properties so that the laminated organic resin film does not peel away from the aluminum surface during drawing.
The method for producing these surface-treated aluminum materials will first be explained using as an example the surface-treated aluminum material employed in a beverage can.
Prior to producing the surface-treated aluminum material, aluminum or an aluminum alloy starting material is first subjected to pre-treating. This pre-treatment is performed in order to remove oil or grease adhering to the surface of the starting material, as well as remove any heterogeneous oxide film on the surface. A step using alkali washing or the like is employed.
Next, the aluminum or aluminum alloy starting material is soaked in a solution comprising mainly phosphoric acid, chromic acid and a fluoride, or a solution comprising mainly chromic acid and a fluoride. An amorphous film of chromate-phosphate or chromate in the form of a thin gel is formed to the surface of the aluminum starting material, thereby obtaining a surface-treated aluminum material with chromate conversion coating.
An organic resin film is then laminated to the surface of the aluminum material that was surface-treated with chromate-phosphate or the like. Two-layer film or the like is frequently employed as the organic resin film, this two-layer film having polyester and an adhesive layer formed by adding a third component to polyester to lower its melting point. This organic resin film is laminated to the surface of a chromate-phosphate film by adhering the low melting temperature adhesive layer. The film is passed through a roller that has been heated to a temperature above the melting point of the adhesive layer, to laminate the organic resin film, thereby obtaining the surface-treated aluminum material. This chromate-phosphate or chromate conversion coating makes it possible to increase the adhesion of the organic resin film to the surface-treated aluminum material.
The thus-obtained aluminum material onto which the organic resin film was laminated does not require coating of a processing lubricant when working the aluminum material. Thus can production costs can be reduced. Moreover, since the material inside the can does not come into direct contact with the aluminum, it is possible to prevent an aluminum odor from tainting the beverage. In addition, management of the treatment bath is easily accomplished for the chromate-phosphate treatment. Thus, working efficiency is high and continuous processing is possible. Accordingly, this process is wildly used.
However, conventional surface-treated aluminum materials in which an organic resin film is laminated over a chromate-phosphate or chromate conversion coating have not sufficient adhesive strength between the aluminum material and the organic resin film. This is because in an extremely thin organic resin film, molecules around the size of moisture that are in the air permeate the organic resin film over a long period of time, and reach the chromate coating. When moisture is present in the coating, this leads to a decrease in the adhesion between the chromate conversion coating and the organic resin film. In addition, the conventional chromate-phosphate or chromate conversion coating is a deposit so that there is low compression. As a result, moisture or corrosive impurities can remain within.
When heat is applied to laminate the organic resin film onto these coatings, the moisture or corrosive impurities in the film, but particularly the moisture, erupts and damages the adhesion between the organic resin film and the film.
The harmful qualities of chrome with respect to environmental contamination have been pointed out, so that use of chromium phosphate and chromium chromate is limited.
A titanium phosphate or zirconium phosphate base layer has been proposed as a non-chrome base treatment. However, when carrying out these base treatments, the adhesive strength between the aluminum material and the organic resin film was less than in the conventional art, and therefore did not provide satisfactory adhesive strength.
A method has also been proposed employing a porous anodic oxide film such as sulfate anodic oxide coating or phosphate anodic oxide coating that takes advantage of the anchoring effect of this porous anodic oxide film to adhere the aluminum material and the organic resin film. However, the diameter of the pores in the anodic oxide film is extremely small, so that the organic resin film does not enter into the pores to any great extent. Thus, a sufficient anchoring effect could not be obtained.
On the contrary, since the aforementioned anodic oxide film is porous, the area of joining with the organic resin film is small so that sufficient adhesive strength is not obtained. Further, moisture remains in the pores even in the case where a pore sealing treatment is performed on the anodic oxide film. Thus, the moisture in the pores erupts when the aluminum material is heated in laminating the organic resin film as described above. As a result, the adhesion between the organic resin film and the aluminum material is inhibited.
The present invention was conceived in view of the above-described circumstances and has as its objective the provision of a surface-treated aluminum material, and production method therefor, that is superior with respect to its adhesive properties with an organic resin film.
The present invention has as a further objective the provision of a surface-treated aluminum material and production method therefor in which the organic resin film is strongly laminated in place.
In order to resolve the above-described problems, a non-porous anodic oxide film is formed to the surface of the aluminum or aluminum alloy in the first surface-treated aluminum material according to the present invention. This non-porous anodic oxide film has a thickness in the range of 5xcx9c800 nm, a porosity of 20% or less, and contains in an amount of 50 ppm or more at least one of silicon, phosphorous, boron or carbon as a component.
It is possible to reduce the amount of moisture contained in the non-porous anodic oxide film by restricting the porosity thereof to be 20% or less. As a result, it is possible to control the amount of moisture erupted from these pores when heating in order to laminate the organic resin film to the surface. Thus, a fixed adhesion between the non-porous anodic oxide film and the organic resin film can be realized.
Further, the silicon, phosphorous, boron or carbon components included in the non-porous anodic oxide film is incorporated into the molecular structure of the organic resin film, thus promoting a strong adhesion between the non-porous anodic oxide film and the organic resin film.
In the present invention, it is preferable that the non-porous anodic oxide film includes silicon or phosphorous component in the amount of 50 ppm or more. These elements are readily incorporated into the molecular structure of the organic resin film as described above, so that a strong adhesion between the non-porous anodic oxide film and the organic resin film is easily achieved.
Specifically, silicon is highly effective in improving adhesive properties. It is preferable that the amount of silicon included is in the range of 100 ppm or more and 20,000 ppm or less.
In the present invention, the moisture content of the non-porous anodic oxide film is 20 wt % or less. When the moisture content of the non-porous anodic oxide film is 20 wt % or less, then it is possible to limit the reduction in adhesion between the organic resin film and the non-porous anodic oxide film due to moisture eruption during heating.
In general, the lower the porosity of the non-porous anodic oxide film, the lower the amount of moisture in the non-porous anodic oxide film. However, the moisture content of the non-porous anodic oxide film can also increase depending on the electrolyzing treatment conditions for forming the non-porous anodic oxide film. Thus, in order to realize a strong adhesion, the moisture content of the non-porous anodic oxide film is 20% or less.
Another aspect of the present invention is a surface-treated aluminum material in which an organic resin film is laminated to the surface of a surface-treated aluminum material as described above. The type of organic resin film for laminating is not particularly restricted. The method of adhesion is not particularly restricted, with thermocompression bonding and the use of adhesive agents being acceptable. Because the anodic oxide film, which serves as the base, is non-porous and has low moisture content, there is no damage from moisture erruption. In addition, chemical bonding between the organic resin film and the non-porous anodic oxide film is promoted via trace quantities of the elements silicon, phosphorous, boron and carbon. Thus, a surface-treated aluminum material is readily obtained in which the non-porous anodic oxide film and the organic resin film are strongly adhered.
A laminate film of polyethylene terephthalate derived resin and copolymer polyethylene terephthalate derived resin films are preferably employed as the organic resin film, for example.
In the present invention, a coating material may be applied to the surface of a surface-treated aluminum material as described above. The type of coating material is not particularly restricted. In this case, the silicon, phosphorous, boron, or carbon included in the non-porous anodic oxide film is also incorporated into the molecular structure of the coating material, thus promoting a strong adhesion between the non-porous anodic oxide film and the coating material.
In the method for producing the present invention""s surface-treated aluminum material, aluminum or an aluminum alloy is subjected to electrolyzation in an electrolytic aqueous solution containing at least one salt selected from the group comprising silicate, phosphate, maleate, benzoate, phthalate, and borate, and a non-porous anodic oxide film is formed to the surface of the aluminum or aluminum alloy that contains a total of 50 ppm or more of at least one type from the group comprising silicon, phosphorous, boron or carbon and has a porosity of 20% or less.
By appropriately selecting the conditions under which the electrolyzation is performed in the electrolyzing aqueous solution containing an appropriate amount of silicon, phosphorous, boron or carbon, a anodic oxide film can be obtained that has a low porosity and contains 50 ppm or more of at least one type of trace component from the group comprising silicon, phosphorous, boron, or carbon. This non-porous anodic oxide film is characterized by its superior adhesive properties with organic resin film.
In another method for producing the present invention""s surface-treated aluminum material, aluminum or an aluminum alloy is subjected to electrolyzation in an electrolytic aqueous solution containing at least one salt selected from the group comprising silicate, phosphate, maleate, benzoate, phthalate, and borate, and a non-porous anodic oxide film is formed to the surface of the aluminum or aluminum alloy that contains a total of 50 ppm or more of at least one type from the group comprising silicon, phosphorous, boron or carbon and has a porosity of 20% or less. This method, however, is provided with the step of then laminating the organic resin film to the surface of the aforementioned non-porous anodic oxide film.
In other words, this is a method for laminating the organic resin film after forming a non-porous anodic oxide film that has superior adhesive properties with the aforementioned organic resin film. As a result, a surface-treated aluminum material is obtained in which the organic resin film is strongly adhered in place. The organic resin film and adhesive method employed to laminate the organic resin film is not particularly restricted. Rather, a variety of resin films and adhesive methods may be suitably used. For example, an adhesive agent or primer may be coated to the non-porous anodic oxide film and pasting carried out.
In the present invention, thermocompression bonding may be used for adhering the organic resin film and the surface-treated aluminum material having the non-porous anodic oxide film. The non-porous anodic oxide film according to the present invention has good adhesion with the organic resin film, so that a strong adhesion can be easily accomplished with thermocompression bonding.
The present invention""s surface-treated aluminum material will now be explained.
The substance employed as the aluminum starting material in the present invention is not particularly restricted. Namely, in addition to pure aluminum, a pure aluminum derived AA1000 alloy (JIS1000 alloy), Alxe2x80x94Mn derived AA3000 alloy (JIS3000 alloy), or an Alxe2x80x94Mg derived AA5000 alloy (JIS5000 alloy) may also be used for example. In addition, it is also acceptable to employ in these alloys a material that has been subjected to various thermal refining treatments such as solution treatment, aging or the like. Any type of rolled plate of these materials is preferably employed. In addition, clad metals of these aluminum alloy may also be used.
Next, a pre-treatment is performed to the aluminum material. This pre-treatment is not particularly restricted, and is for the purpose of removing heterogeneous oxide film from the surface of the aluminum starting material. For example, a method may be employed in which washing is carried out using a alkalescent degreasing solution, followed by alkaline etching in a sodium hydroxide aqueous solution and desmutting treatment in an aqueous solution of nitric acid. Alternatively, another acceptable method is one in which acid washing is carried out after washing using a degreasing solution.
Next, the pre-treated aluminum starting material is subjected to electrolyzation in the electrolytic solution, and anodizing process for forming the non-porous anodic oxide film to the surface of the aluminum starting material is carried out.
For the electrolytic solution, an electrolytic aqueous solution is employed containing an appropriate amount of silicon, phosphorous, boron or carbon, so that the solution does not readily dissolve the generated non-porous oxide film, and so that at least one element from the group comprising silicon, phosphorous, boron and carbon is incorporated in the non-porous oxide film in an amount of 50 ppm or more, and preferably 100 ppm or more.
To obtain this type of electrolytic aqueous solution, an appropriate amount of at least one type of salt from the group comprising silicate, phosphate, borate, maleate, benzoate, phthalate, adipate, tartrate, citrate, and malonate. The amount added will differ depending on the type of salt and the electrolyzing conditions, however, it is roughly in the range of 2xcx9c150 g/l. When the concentration of trace elements is high in the electrolytic aqueous solution, the concentration of trace elements incorporated into the non-porous oxide film also increases.
When the temperature of the electrolytic bath during the anodizing increases, the concentration of trace elements mentioned above that are incorporated into the non-porous anodic oxide film tends to increase. In addition, when the electric current density during the anodizing increases, the temperature of the electrolytic surface rises, and the concentration of trace elements incorporated in the non-porous anodic oxide film tends to increase. A suitable electrolytic concentration should be determined for each salt added based on these trends. The temperature of the electrolytic bath is in the range of 30xc2x0 C.xcx9c60xc2x0 C., the current density is in the range of 0.3xcx9c10 A/dm2, and the applied voltage is in the range of 1.4xcx9c570 V. The electrolyzing conditions are appropriately set within these ranges, so that the porosity of the anodic oxide film is 20% or less, and preferably 10% or less, and the silicon, phosphorous, boron or carbon component contained in the anodic oxide film is in the amount of 50 ppm or more, and preferably 100 ppm or more. An important point to remember when carrying out the anodizing is to avoid formation of pores by gradually forming the anodic oxide film.
The thickness of the non-porous anodic oxide film is adjusted depending on the electrolyzing time, but is typically in the range of 5xcx9c800 nm, and preferably 50xcx9c200 nm.
The anodic oxide film obtained using this type of anodizing is a non-porous anodic oxide film of uniform thickness. A thickness in the range of 5xcx9c800 nm is acceptable for the non-porous anodic oxide film. When the film thickness is 5 nm or less, then this is about the thickness of the oxide film that naturally forms on the aluminum surface and resistance to corrosion cannot be obtained. On the other hand, when the thickness exceeds 800 nm, defects are readily generated in the film, so that the film is fractured and durability deteriorates. Accordingly, the thickness of the non-porous anodic oxide film is appropriately in the range of 5xcx9c800 nm, and preferably in the range of 50xcx9c200 nm.
It is acceptable for the non-porous anodic oxide film to have a degree of porosity that is 20% or less. Intermetallic compounds between aluminum and trace impurities such as silicon or iron are present on the surface of the aluminum or aluminum alloy. Since it is difficult to generate an anodic oxide film on top of these impurities, very small pores are generated in the anodic oxide film. Areas where these impurities are not exposed are essentially non-porous. In the anodic oxide film, it is acceptable if the pore surface area ratio, which is calculated by adding all the pores arising from these types of impurities, is 20% or less. Note that xe2x80x9c20% or lessxe2x80x9d includes a pore surface area ratio of 0%. The degree of porosity of the non-porous anodic oxide film can be measured by enlarging the surface 100,000 fold using an electron microscope and examining the surface, and then calculating the surface area of the pores.
A non-porous anodic oxide film having few such pores has a low amount of impurities of a corrosive nature, chief among these being water, remaining in the pore. Thus, even when heating during the step to laminate the organic resin film to the surface of the non-porous anodic oxide film, there is no eruption of water from the surface of the anodic oxide film.
Further, in the case of a porous film such as sulfate anodic oxide film, when a pin hole occurs in the organic resin film, corrosive material can penetrate via the pin hole to the base aluminum material and corrode it. However, the present invention""s non-porous anodic oxide film has high barrier properties, so that it is possible to avoid corrosion of the base aluminum starting material.
In addition, the moisture content of the present invention""s non-porous anodic oxide film is less than 10%, and preferably 5% or less, for the film as a whole, with 2% or less being optimal. The moisture contained in the anodic oxide film erupts from the pore when heating during the step to laminate the organic resin film to the surface of the non-porous anodic oxide film, impairing the adhesive properties between the organic resin film and the anodic oxide film. For this reason, it is desirable that the moisture content of the non-porous anodic oxide film be extremely low. The moisture content of the non-porous anodic oxide film is the amount of moisture in the film as a whole, and comprises not only the moisture adhering to the surface, but also the moisture, which has permeated into deep areas within the pores and crystallization water.
The moisture content of the non-porous anodic oxide film is measured using a thermogravimetric method in which the film is heated to 500xc2x0 C. and the decrease in mass is measured.
When the moisture content of the non-porous anodic oxide film exceeds 5%, the adhesive properties with organic resin film decrease. It is of course desirable that the moisture content of the non-porous anodic oxide film be 0%, however, it is difficult to actually achieve this value. Thus, there is no impairment to the film as long as the moisture content in the non-porous anodic oxide film is 5% or less, while a moisture content of 2% or less enables firmer adhesion between the anodic oxide film and the organic resin film.
It is acceptable for the present invention""s non-porous anodic oxide film to contain in an amount of 50 ppm or more at least one of silicon (Si), phosphorus (P), boron (B) or carbon (C). Both a physical adhesive strength (chiefly from the anchoring effect) and a chemical adhesive strength (chemical bonds, H bonds, van der Waals forces) contribute to the strength of adhesion between the anodic oxide film and the organic resin film. It was discovered that chemical adhesive strength was promoted when Si, P, B or C was present in the anodic oxide film in an amount of 50 ppm or more. In order to promote this chemical adhesive strength, it is preferable to include the aforementioned Si, P, B or C in an amount of 50 ppm or more, and more preferably 100 ppm or more. From among the elements listed above, silicon (Si) and phosphorous (P) have a large effect, with the efficacy of silicon (Si) being particularly remarkable. For silicon (Si), there is a considerable effect when it is present in a range of 100 ppm or more to 20,000 ppm or less.
Further, the above-described effects can be obtained as long as P, B or C are contained in an amount of 50 ppm or higher. In particular, a marked efficacy is seen when P and B are in the range of 20,000 ppm or less and C is in the range of 50,000 ppm or less.
In order to obtain a non-porous anodic oxide film containing these components in an appropriate amount, an electrolytic solution containing these components in the appropriate amount as described above is used, the electrolyzing conditions are appropriately selected, and anodic oxidation is carried out.
In addition, an organic resin film is laminated to the surface of the aluminum material on which the non-porous anodic oxide film containing the above element in an appropriate amount is formed. The organic resin film used is not particularly restricted, with polyethylene (PE) resins, polyethylene terephthalate (PET) resins, polypropylene (PP) resins and polyamide resins (nylon, etc.) being applicable. It is acceptable to employ a laminated polyethylene terephthalate resin film with a copolymer polyethylene terephthalate resin film in which the melting point is lowered by copolymerizing a third component with polyethylene terephthalate resin. The adhesive method is not particularly restricted, with thermocompression bonding or employment of an adhesive agent being acceptable.
The laminating treatment is not particularly restricted. For example, a suitable resin film may be prepared and closely contact to the non-porous anodic oxide film formed of the surface-treated aluminum material having the non-porous anodic oxide film that was obtained by the method described above. This laminate is then passed through a roller that has been heated to a temperature above the melting point of the organic resin film, and the organic resin film is laminated to the anodic oxide film. The pressure applied to the roller, etc. is appropriately determined based on the material and thickness of the organic resin film.
In the thus-obtained surface-treated aluminum material onto which the organic resin film is laminated, the aluminum starting material and the organic resin film are adhered in place. As a result, even when the material undergoes further processing, the organic resin film does not peel away. In addition, since the non-porous anodic oxide film is present at the base of the organic resin film, the barrier property is excellent and resistance to corrosion does not deteriorate over time. As a result, the present invention""s surface-treated aluminum material can be widely employed in various containers, chief among these being beverage cans, caps, and food containers, as well as deep-drawing containers and decorative boards.
Effects of the Invention
It is the objective of the present invention to promote adhesive strength when laminating an organic resin film by taking advantage of the low porosity and moisture content in a non-porous anodic oxide film obtained by an anodic oxidation treatment under specific conditions. Moreover, the present invention promotes adhesion between the non-porous anodic oxide film and the organic resin film by including elements that readily give rise to specific chemical compounds in a non-porous anodic oxide film.